This invention relates to single component developer compositions, and more specifically to single component resistive toner compositions containing magnetic materials therein. In one embodiment, the present invention is directed to magnetic single component resistive toners with islands or patches of carbon black on the surface thereof. These toner compositions are particularly useful in electrostatographic imaging systems wherein an offset preventing liquid, such as a silicone oil is not required.
Developer compositions for use in electrostatographic imaging processes are well known, these compositions generally being comprised of resin particles, pigment particles, and carrier particles. Many of the prior art developer compositions also contain therein various additives, such as fatty acid salts, Aerosil, and other similar materials. There is, for example, described in British Pat. No. 1,442,835 developer compositions with polystyrene resin particles, pigment particles, at least one polyalkylene compound selected from polyethylene and a polypropylene, and as an optional ingredient a paraffin wax and a metal salt of a fatty acid. According to the disclosure of this patent, the addition of a metal salt of a fatty acid provides for the improved compatibility of the polyalkylene compound in the resin component. Also, apparently the dispersion capability of the paraffin wax, pigment particles, and other toner additives, such as charge enhancing additives, is further improved with the use of zinc stearate. For example, it is stated in column 4, beginning at line 45 of this patent that; (1) the non-sticking property of the resulting toner particles are improved; (2) the stability of the toner composition involved can be marketedly enhanced; and (3) the toners life can be sharply prolonged without being subjected to a change in frictional charge characteristics, even when the toner is used for a long period of time. Furthermore, it is indicated in this patent that the resulting toner compositions may also be improved in their moisture resisting properties.
Disclosed in U.S. Pat. No. 3,320,169 is a developer composition comprised of three components, namely magnetic carrier particles, toner particles consisting of a discrete mixture of pigmented resin particles, and an aliphatic acid having from about 10 to 26 carbon atoms, and/or salts of such aliphatic acids. Examples of fatty acids disclosed include saturated or unsaturated acids containing from 10 to 26 carbon atoms such as luric, stearic, oleic, and the like. Preferred additives are calcium stearate and lithium stearate. Examples of thermoplastic resin particles disclosed in this patent include polystyrene resins, acrylic resins, asphalt, polyvinyl resins, and the like.
Further, there is disclosed in numerous prior art patents magnetic toner compositions comprised of a major amount, in excess of 40 percent by weight, of magnetite particles. Thus, for example, there is disclosed in U.S. Pat. No. 3,639,245, a dry toner powder having a specific electric conductivity and containing magnetite particles which are blended with a toner resin, followed by pulverizing the resulting mixture to small particle sizes. Thereafter, the particles are mixed with carbon black and small particle silicone dioxide particles for the purpose of improving flowability. The toner compositions of the '245 patent are conductive, that is, they are of a resistivity of at most 10.sup.11 ohm/cm. There is also disclosed in Xerox copending applications magnetic toner composition comprised of toner resin particles, such as styrene butyl acrylate copolymers, and magnetite particles, in an amount of from about 40 percent by weight to about 70 percent by weight. More specifically, for example, there is disclosed in copending application U.S. Ser. No. 227,003 conductive single component magnetic dry toner compositions comprised of resins selected from polystyrene, allyl alcohol copolymers, and epoxy resins, a magnetic material such as magnetite, and a conductive carbon black which is adhered and/or embedded on the surface of the copolymer or epoxy resin magnetic material mixture. This adherence can be accomplished by a number of known methods, including heat spheroidization. The magnetic particles are present in the developer composition in an amount of from about 40 percent by weight to about 60 percent by weight, and preferably about 50 percent by weight.
Moreover, it is known that in order to substantially eliminate offsetting, and more specifically for the purpose of preventing adhesion of the toner particles to the surface of fixing rollers, there can be selected certain types of rollers the surface of which may be covered with a thin film of an offset preventing liquid, including silicone oils. These oils are highly effective however, the apparatus within which they are incorporated is complicated and costly since, for example, a means for feeding the oil is required. Also, not only do the silicone oils emit an undesirable odor, they have a tendency to deposit on the machine components causing toner particles to collect thereon and adhere to the silicone oils, which is highly undesirable. An accumulation of toner particles on machine components is troublesome in that the image quality is effected, and further these components must be periodically cleaned and/or replaced, adding to the maintenance cost of the machine system involved.
There is a continuing need for improved magnetic toner compositions, particularly those of specific resistivities enabling more effective transfer of the developed image. Moreover, there continues to be a need for single component resistive toner compositions which are useful in electrostatographic imaging devices wherein offset preventing liquids, such as silicone oils are not required. Furthermore there remains a need for improved resistive single component developer compositions which are of sufficient conductivity to enable the development of high solid areas. Also there is a need for single component magnetic toner compositions wherein the resistivity remains stable over relative humidities of from about 40 percent to about 90 percent. Additionally, there continues to be a need for single component resistive toner compositions which are particularly useful in a xerographic imaging process where fusing oils are eliminated.